Collective charge density fluctuations in superconducting layered systems with bilayer unit cells

TL;DR

This paper investigates collective charge density fluctuations in layered superconductors with bilayer units, revealing how interlayer tunneling influences plasmon modes and their experimental signatures.

Contribution

It provides a theoretical analysis of collective modes in bilayered superconducting superlattices, highlighting the effects of tunneling on mode dispersion and spectral weight.

Findings

Out-of-phase mode develops a tunneling-dependent plasmon gap.

Oscillator strength of out-of-phase mode dominates at certain wave vectors.

Mode dispersions are insensitive to the tunneling mechanism.

Abstract

Collective modes of bilayered superconducting superlattices (e.g., YBCO) are investigated within the conserving gauge-invariant ladder diagram approximation including both the nearest interlayer single electron tunneling and the Josephson-type Cooper pair tunneling. By calculating the density-density response function including Coulomb and pairing interactions, we examine the two collective mode branches corresponding to the in-phase and out-of-phase charge fluctuations between the two layers in the unit cell. The out-of-phase collective mode develops a long wavelength plasmon gap whose magnitude depends on the tunneling strength with the mode dispersions being insensitive to the specific tunneling mechanism (i.e., single electron or Josephson). We also show that in the presence of tunneling the oscillator strength of the out-of-phase mode overwhelms that of the in-phase-mode at $k_{\|} = 0$ and finite $k_z$, where $k_z$ and $k_{\|}$ are respectively the mode wave vectors perpendicular and along the layer. We discuss the possible experimental observability of the phase fluctuation modes in the context of our theoretical results for the mode dispersion and spectral weight.